Cavity resonator device for production of high-speed electrons



Patented Feb. 11;.

o s'rATEs OFFICE CAVTY RESONATOR DEVICE FR. PRDUC- TIN F HIGH-SPEED ELECTRONS (Granted under the act of March 3, 1883., as amended April 30, 1928i; 370 0. G. 757) li) Claims.

l invention described herein may be manuactured and used by or for the Government for ovenmental purposes, without the payment to me of any royalty thereon.

This invention relates to the production of electrons with speeds equal to those produced by electrostatic fields of millions of volts. At present, the chief use of such high speed electrons is for the production of very hard X-rays. Such rays are used mainly for cancer therapy; but also they can be used in important research problems and for the examination of metal castings and other objects too thick for less penetrating X-rays.

It is an object of this invention to provide a dei/'ice for producing high speed electrons which requires only relative low potentials as compared to certain prior devices.

A further object is the provision o a method and apparatus for producing high speed electrons efficiently and easily than has heretofore been possible.

rhese and other objects are attained by the novel method and arrangement of apparatus hereinafter described and briefly speaking, one iorm of the invention resides in the combination of a plurality of cavity resonators the energizing fields oi which are established by a pulsed magnetron and arranged whereby the cavity resonators successively increase the speed and mass of the electrons passing there through,` illustrated by the accompanying drawing forming a part ofl this disclosure. The gure is a schematic diagram showing the arrangement of apparatus in accordance with the invention.

Referring to the block diagram in the accompanying drawings, there are shown sixteen cavity resonators designated by reference numbers lY thru it respectively, all alike and all excited in the same phase by a pulsed magnetron Il, through coaxial transmission lines of equal lengths, made by branching in the same Way as for the excitation oi a synchronous antenna array.

The tubes i8 projectingv intov and connecting adjacent cavity resonators are of graded lengths depending upon the speeds of the electrons passing through them. The tubes are all grounded; and the fields between the tubes are of the electrmdynainic type such as cavity resonators, for example and do not involve high-Voltage insulaition. The electron gun shown diagrammatically at l has a cathode 23 which is charged to a high potential, and is kept negative by a direct current source 3l. Its emission current is cut oli by the grid 2d except during the pulses of the magnetron,

when synchronous pulses on the grid permit it to .shoot electrons into the rst resonator. Some of these electrons, arriving in the Wrong phase7 are slowed down in this resonator and give up energy to it; but those which arrive in the right phase are speeded up by it.

if the field strengths and distances are properly proportioned, these speeded-up electrons Will enter the tube leading to the second resonator before the end of the half cycle of electric eld helping them along. In this tube there is no appreciable eld, so the electrons spend the next half cycle drifting through eld free space in the connecting tube, during which time retarding nelds arise in the first resonator. They then emerge into the second resonator during the first hah" of the next cycle of helping field. They repeat this sort of hiding from hindering fields and coming out into helping nelds, all the Way v through the line of resonators.

To see how the lengths of the connecting tubes should be proportioned, it must be noted that at thesev very high speeds the ordinary relation of speed to kinetic energy does not hold. Instead, if 'u is the speed of an electron and; c is the speed where V0=509 `klovolts and V measures the kinetic energy, being the potential dii-ference needed in a single field to get this energy. Vu is twice the potential difference which would give an electron the speed of light if ordinary laws of mechanics extended to such speeds; but from this formula, whenIV=Vo, o2 is still only 3A of c2, or o=0.87c. No matter how high V may be, o is always less than c.

For a typical case, assume that the potential of the cathode is Vo/2, or -255 kv., and that each cavity is` excited so strongly that its eld gives an electron as strong a push as the cathode-toground field. (It cannot be said that the cavity has the same potential difference, because the laws of potentials do not apply to cavity-resona tor fields; so by push is meant the lineintegral of the field through theY cavity.) Under these conditions, the electron leavesV the first cavity at the same speed as if it has fallen through a potential dierence Vo, namely 0.870 and this is the speed in the rst connecting tube.

Ink the last connecting tube, after 14 more llo/2 pushes, the kinetic energy is equivalent to a fall through a. potential di'iferenee 8vo; This makes :02930. So the last connecting tube must be long enough to space the centers of the resonators it connects 9%?, or 1.14 times as far apart as the centers of the rst and second cavities. The other distances on centers are, of course, intermediate between these, most of them being more nearly equal to the last than to the first.

To keep the lengths of the concentric lines from the magnetron all equal, the lines may be curved as shown in the diagram, if they all lie in one plane, or the longer ones may be taken out of this plane. With speeds such as herein considered, the phasing should be accurate enough if only the i-lrst 3 or 4 of lines 22 are curved and the rest straight and coplanar.

Under the assumed conditions the electron emerges from the last cavity with the same energy as if it had fallen through a potential of 8.5Vo, or 4.3 million volts. With this energy it strikes the anode block 2l, or target, as it is called in X-ray technique, and produces hard X-rays.

Many changes in the dimensions, frequency, voltages, and even the number of resonators, are possible, of course.

The magnetron may be operated at 200 megacycles with high frequency power between 100 and 200 kilowatts during pulses. When 1200 kilowatts are available there is an ample margin for building up the oscillations in each pulse and for driving the free electrons.

The tubes are made too small in diameter to act as wave guides and therefore they act as shields. The apertures projecting into the resonators set up fields which tend to keep the beam of electrons traveling in columns. It is of course possible to initiate flow of electrons in columns by use of electron guns which are well known in the art of radio. Cavity resonators 1 thru 16 are likewise well known in the radio art and are used in the production of microwaves.

For typical dimensions, if the frequency used is 200 Inc., as in the example described above, the lengths of the drift tubes may be as follows:

Drift Tube Resonators Lengths Cm. 1 and 2 64.6 2 and 3 68.8 3 and 4 70. 8 4 and 5 7l. 9 5 and G 72. 6 6 and 7 73. 1 7 and 8 73. 5 8 and 9 73. 8 9 and 10 74. 0 10 and 1l 74.1 l1 and 12 74. 2 l2 and 13 74. 3 13 and 14 74. 4 14 and 15 74. 5 15 and 16 74.5

where the values apply to the case described above in which 1/2 cycle is allowed for the electrons to pass through each drift tube and 1/2 cycle to pass through each resonator.

Care must be taken, as noted above, to have the resonators all respond alike to the exciting frequency. The gaps between the ends of the connecting tubes may all be equal and the lengths of the drift tubes must be selected to have a time of transmit equal to that part of the cycle not occupied by electron passage through the gaps in the resonators. If the gaps are long as in the example just described, an allowance must be made, in estimating the push dened above, for the fact that each electron takes a finite time to cross each gap. During this time the iield and its line integral change, so the push" is an average of the values the line integral which in the 1/2 cycle example given above is 1/2 the maximum value. If now, for example, the gaps are made of such length that an electron in some typical resonator takes only one sixth of a cycle to cross its gap, and it is desired to give it a push of 255 kv. in that cavity, the maximum value of the line integral will be 5% greater than 255 kv., or about 268 kv. The longer the gaps are, the more the voltage has to be raised for this cause, to get any required push, but with this understanding any gap length of this order of magnitude may be used.

The first example given, wherein 1/2 cycle is allowed for the passage of electrons through the drift tubes and 1/2 cycle for passage through each resonator is intended for purposes of illustration and to facilitate description of the essential features of the device. In the second example, which is the preferred embodiment of the in- Vention, the resonators are all substantially alike. The drift tubes project through the end walls of the resonators to provide a gap within each resonator in which electron acceleration takes place. The time of transit of electrons across each gap is made much shorter than the time of 1/2 cycle, preferably 1/6 cycle. During this relatively short interval of electron transit through each gap the line integral of electric eld intensity is substantially a maximum. The value of this maximum required for a given electron push is therefore much less than for the 1/2 cycle example, and offers an advantage in obtaining a large output per resonator. The time of electron passage through the drift tubes must be correspondingly longer, that is, of the order of cycle. The length of each drift tube, which is proportional to the time of drift, is correspondingly longer. In this example the length of the gap in each resonator may be 20 cm. and the length of the first drift tube may be 108 cm. as compared to 64.6 cm. given in the table. The length of the last drift tube may be 124 cm. as compared with 74.5 cm. The lengths oi the intermediate drift tubes increase progressively from 108 cm. to 124 cm. after the manner shown in the table.

Having described my invention, what is claimed 1s:

1. An accelerator for producing very high velocity electrons comprising, a sequence of evacuated cavity resonators assembled on a common axis with intervening spaces between resonators, drift tubes in the intervening spaces connecting adjacent resonators means adjacent said cavity resonators for injecting electrons into the irst resonator, and means for exciting axial high intensity electric fields within the resonators, said exciting means including a source of high frequency oscillations and transmission lines connected to said source and terminating in the individual resonators, said lines having equal lengths whereby all resonators are excited in the same phase, and electrons are accelerated within the resonators during one half cycle of the exciting frequency and drift between resonators during the next half cycle, said accelerative eiects being successive and cumulative.

2. An accelerator for producing high energy charged particles comprising a sequence of evacuated cavity resonators assembled on a common axis with intervening spaces between resonators,

asiatica drift tubes in the intervening spaces connecting adjacent resonators, means adjacent said cavity resonators for injecting charged particles in the iirst resonator, and means for exciting axial high intensity electric fields Within the resonators, said exciting means including a source of high frequency oscillations and transmission lines connected to said source and terminating in the individual resonators, said lines having equal lengthswhereby all resonators are excited in the same phase, and said charged particles are accelerated within the resonators during one half cycle of the exciting frequency and drift between resonators during the next half cycle, said accelerative eects being successive and cumulative.

3. An accelerator for producing very high velocity electrons comprising, a sequence of evacuated cavity resonators assembled on a common axis with intervening spa-:ies between resonators, drift tubes in the intervening spaces connecting adjacent resonators, means adjacent said cavity resonators for injecting electrons axially into the first resonator, and means for exciting axial high intensity electric iields within the resonators, said exciting means including a source of high frequency oscillations and transmission lines connected to said source and terminating in coupling loops within the individual resonators, said lines having equal lengths whereby all cavities are excited in the same phase, said cavity resonators and said drift tubes each having an electron transit time through each resonator and through each drift tube substantially equal to a half cycle period of the exciting frequency, whereby electrons,v are accelerated within the resonators during one half cycle and drift between resonators during the next half cycle, said accelerative eiects being successive and cumulative..

4. An accelerator for producing very high velocity charged particles comprising, a sequence of evacuated cavity resonators assembled on a common axis with interveningspaces between resonators, drift tubes in the intervening spaces connecting adjacent resonators, means adjacent said cavity resonators for injecting charged particles axially into the first resonator, and means for exciting axial high intensity electric elds within the resonators, said exciting means including a source of high frequency oscillations and transmission lines connected to said source and terminating in coupling loops within the individual resonators, said lines having equal lengths whereby all resonators are excited in the same phase, said cavity resonators and said drift tubes each having a particle transit time through each resonator and through each drift tube substantially equal to a half cycle period of the exciting frequency, whereby said charged particles are accelerated within the resonators during one half cycle and drift between resonators during the next half cycle, said accelerative effects being successive and cumulative.

5. An accelerator for producing very high speed electrons comprising, a sequence of cylindrical cavity resonators having progressively increasing lengths assembled on a common axis with intervening spaces between resonators, drift tubes in the intervening spaces having progressively increasing lengths connecting adjacent resonators, means adjacent said cavity resonators for injecting high speed electrons axially into the first resonator, means adjacent the last resonator d for utilizing the electrons issuing therefrom and means for exciting axial high intensity electric iields within the resonators, said exciting means including a source or pulsed high frequency oscillations and coaxial transmission lines connected to said source and terminating in coupling loops within the individual resonators, said lines having such lengths that allcavities are excited in the same phase, said cavity resonators and said drift tubes each having an electron transit time through each resonator and through each drift tube substantially equal to a half cycle period of the exciting frequency, and electrons are accelerated within the resonators during one half cycle and drift between resonators during the next half cycle, said accelerative effects being successive and cumulative.

6. An accelerator for producing very high speed charged particles comprising, a sequence of cylindrical cavity resonators having progressively increasing lengths assembled on a common axis with intervening spaces between resonators, drift tubes in the intervening spaces having progressively increasing lengths connecting adjacent resonators, means adjacent said cavity resonators for injecting high speed particles axially into the rst resonator, means adjacent the last resonator for utilizing the particles issuing therefrom and means for exciting axial high intensity electric fields within the resonators, said exciting means including a source of pulsed high frequency oscillations and coaxial transmission lines connected to said source and terminating in coupling loops within the individual resonators, said lines having equal lengths whereby all resonators are excited in the same phase, said cavity resonators and said drift tubes each having a particle transit time through each resonator and through each drift tube substantially equal to a half cycle period of the exciting frequency, and said charged particles are accelerated within the resonators during one half cycle and drift between resonators during the next half cycle, said accelerative effect being successive and cumulative.A

7. An electron accelerator for producing high speed electrons comprising a sequence of cylindrical cavity resonators having progressively increasing lengths assembled on a common axis With intervening spaces between resonators, drift tubes in the intervening spaces having progressively increasing lengths connecting adjacent resonators, means adjacent said cavity resonators for intermittently injecting high speed electrons axially into the rst resonator, means for exciting axial high intensity electric elds within the resonators, and a target adjacent the last resonator upon which high speed electrons impinge, said exciting means including a source of pulsed high frequency oscillations and coaxial transmission lines connected to said source and terminating in coupling loops within the individual resonators, said lines having equal lengths whereby all resonators are excited in the same phase, said cavity resonators and said drift tubes each having an electron transit time through each resonator and through each drift tube substantially equal to a half cycle period of the exciting frequency, and electrons are accelerated within the resonators during one half cycle and drift between cavities during the next half cycle, said accelerative effects being successive and cumulative.

8. An accelerator for producing very high velocity electrons comprising. a sequence of similar evacuated cavity resonators assembled Dn a amaca@ common axis with intervening spaces between resonators, drift tubes in the intervening spaces connecting adjacent resonators, means adjacent said cavity resonators for injecting electrons into the rst resonator, and means for exciting axial high intensity electric fields within the resonators, said exciting means including a source of high frequency oscillations and transmission lines connected to said source and terminating in the individual resonators, said lines having equal lengths whereby all resonators are excited in the same phase, said resonators having an electron transit time therethrough less than one half cycle of the exciting frequency, each following drilt tube having a time of drift therethrough occupying the remaining part of the cycle, whereby electrons are alternately accelerated in the resonators and drift through the iollowing drift tubes, the accelerative eiects being cumulative.

9. An accelerator for producing very high velocity charged particles comprising, a sequence of similar evacuated cavity resonators assembled on a common axis with intervening spaces between resonators, drift tubes in the intervening spaces connecting adjacent resonators, means adjacent said cavity resonators for intermittently injecting charged particles axially into the rst resonator, and means for exciting axial high intensity electric fields within the resonators, said exciting means including a source of high frequency oscillations and transmission lines connected to said source and terminating in coupling loops within the individual resonators, said lines having equal lengths whereby all resonators are excited in the same phase, said cavity resonators having a particle transit time therethrough of less than one half cycle of the exciting frequency, each following drift tube having a time of drift therethrough occupying the remaining part of the cycle, whereby the particles are alternately accelerated in the resonators and drift through the following drift tubes, the accelerative eects being cumulative.

10. An electron accelerator for producing high speed electrons comprising, a sequence of identical cylindrical cavity resonators assembled on aV common axis with intervening spaces between resonators, drift tubes in the intervening spaces having progressively increasing lengths connecting adjacent resonators, means adjacent said resonators for intermittently injecting high speed electrons axially into the rst resonator, means for exciting axial high intensity electric fields within the resonators, and a target adjacent the last resonator upon which high speed electrons impinge, said exciting means including a source of pulsed high frequency oscillations and coaxial transmission lines connected to said source and terminating in coupling loops within the individual resonators, said lines having equal lengths whereby all resonators are excited in the same phase, said resonators and said drift tubes each having an electron transit period through each resonator occupying a small part of a cycle of the exciting frequency and a transit period through the following drift tube occupying the remaining part of the cycle, said accelerative eiects being successive and cumulative.

DAVID L. WEBSTER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,009,457 Sloan July 30, 1935 2,245,670 Hollmann June 17, 1941 2,250,698 Berline July 29, 1941 2,275,480 Varian et al Mar. 10, 1942 2,280,824 Hanson et al Apr. 28, 1942 2,281,935 Hansen et al. May 5, 1942 2,284,751 Linder June 2, 1942 2,304,186 Litton Dec. 8, 1942 2,305,883 Litton Dec. 22, 1942 2,317,140 Gibson Apr. 20, 1943 2,464,349 Samuel Mar. 15, 1949 

